1. Field of the Invention
This invention relates to a thick film thermistor composition.
2. Description of the Prior Art
Heretofore, thick film thermistors have been formed according to the ordinary thick film technique comprising steps of screen printing a thermistor paste comprising powders having a thermistor characteristic, glass powder, and an organic vehicle on an insulating substrate, firing, etc., and their structures can be classified into two main groups: thick film resistor type structure (which will be hereinafter referred to as a sheet type) and thick film condenser type structure (which will be hereinafter referred to as a sandwich type). Thermistor materials having a high stability now employed in the bead-form thermistor or disc-form thermistor, or the like have a high specific resistance, for example, 500 .OMEGA.-cm or higher, and the glass itself has a very high specific resistance. Thus, when a thick film thermistor is prepared from these materials, the structure is always of sandwich type which naturally provides a low resistance. The sandwich type thick film resistor is thus applied to the ordinary electric circuit.
However, the sheet type thermistor has more advantages such as a low cost, a high reliability, etc. in the process for producing thick film thermistors and the structure than the sandwich type thermistor, because of less processing steps, wide interelectrode distance, etc. That is, the sheet type thermistor is industrially more advantageous than the sandwich type thermistor, if the thermistor film itself can be made to have a low resistance. To this end, two methods can be expected: (i) an electroconductive powder is added to a thick film thermistor composition, and (ii) a material having a low resistance is used as the thermistor powder itself. However, when the electroconductive powder is added to the composition according to said method (i) until the resistance of the sheet type thermistor becomes less than 10 k.OMEGA., the thermistor constant is decreased to less than one-half of the thermistor constant of the thermistor powder itself. Thus, it is very difficult to prepare a sheet type thermistor having such a characteristic that a thermistor constant is more than 2,000 K, while the thermistor has the practical resistance. According to said method (ii), a sheet type thermistor having the desired characteristics can be prepared, using thermistor powders having a specific resistance of less than 100 .OMEGA.-cm and containing Cu. However, the thermistor material containing Cu has a problem in stability, and a large change in resistance, and thus cannot be used as a heat-sensitive element of high precision.
Compound metal oxides of pyrochlore (compound oxides of Cd, Bi, Nb, and Ru) are known as a thermistor material containing an oxide of Ru as a noble metal, but require firing at 1,200.degree. C. for 16 hours (Japanese Laid-Open Patent Application Specification No. 118295/75).